Arizona BAC for a DUI (and how it’s calculated)

If you get caught driving a personal vehicle with a blood alcohol concentration (BAC) level at or over 0.08%, you will be arrested for driving under the influence (DUI).1 Of course, there is a zero tolerance for any level of alcohol found in your blood if you are under 21 because you are under the legal drinking age.2 If you are a minor charged with a DUI you face hefty charges under the zero tolerance policy, such as license suspension and possible additional criminal penalties.3 The percentage of alcohol in your blood that will lead to liability is more strict if you are driving a commercial vehicle; if you are caught with your BAC at 0.04% you will be charged with a DUI.4

The science behind how your BAC is measured is relatively simple. It is measured by dividing grams of alcohol by 100 milliliters of blood.5 You should not approach these calculations by asking how much it takes to become legally drunk. This is because it is often difficult to estimate what your BAC is without some type of test, as everybody has a different body type and tolerance level. Additionally, time and the type of alcohol have a role to play; for example, 12 fluid ounces of beer and five fluid ounce glasses of wine are calculated as one drink in a standard BAC calculator.6 Typically, though, alcohol is absorbed relatively quickly (within minutes) and reaches its highest level about an hour after you drink it.7

You can determine BAC through three types of tests: (1) blood, (2) breath, (3) urine.8 You do not have a choice about which test the police officer will choose.9 You are presumed to give consent to any of these chemical tests in Arizona.10

Blood

You can measure the amount of alcohol in your blood easily through a blood test, which usually involves taking a blood sample from a vein in your arm.11 However, some studies have shown blood draws can lead to false positives up to about half the time.12 This may be a result of the blood sample specimen becoming compromised for some reason or another.13 The storage of the blood sample (both in the amount of time and where the sample is stored) can also contaminate the sample.14 Still, this is generally the more accurate test when determining BAC and usually requires your consent before it is performed.15

Breath

By far the most common method used by police officers is the breath test because breathalyzers are portable and provide immediate results.16 The breathalyzer determines how much alcohol passed through your alveoli (the air sacs in your lungs) and then is expelled by your breath.17 A problem with this type of testing is it considers everyone to be the same and averages results, which can be inaccurate depending on the type of individual you are.18 Thus, sometimes even if you do not feel intoxicated you may be surprised if you have a BAC above 0.08% with a breath test.19

Urine

While the urine test is less accurate than both the breath and blood tests, it is used as a last resort.20 One of the most popular types of urine tests is the ethyl glucuronide (EtG) test because it can determine recent alcohol consumption in your urine even immediately after you consumed alcohol.21 This is important because (1) you usually cannot take a urine test at the moment of the police stop and (2) alcohol stays in your urine for six to twenty-four hours but it takes about two hours to typically appear in your urine.22 In contrast, EtG, a direct metabolyte of alcohol, can stay in your system for about 80 hours.23

However, like any test, EtG can lead to false positives and has been criticized by the Substance Abuse and Mental Health Services Administration.24

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